I have this plugin installed on many of my sites -- even those with responsive themes! That's because there can be situations when even a responsive theme's small-screen layout doesn't look as good as a simple non-responsive one's..."WP Themes by Screensize" is one of the most useful free plugins I know of and it definitely works with 3.5.1 despite the lack of continued development. But hey, it's simple and lightweight so it should withstand many more updates to come!!